Proteomic identification of proteins translocated to membrane microdomains upon treatment of fibroblasts with the glycosphingolipid, C8-β-D -lactosylceramide

Effect of cholesterol on the lactosylceramide domains in phospholipid bilayers

Lactosylceramide (LacCer) in the plasma membranes of immune cells is an important lipid for signaling in innate immunity through the formation of LacCer-rich domains together with cholesterol (Cho). However, the properties of the LacCer domains formed in multicomponent membranes remain unclear. In this study, we examined the properties of the LacCer domains formed in Cho containing 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) membranes by deuterium solid-state NMR and fluorescence lifetimes. The potent affinity of LacCer-LacCer (homophilic interaction) is known to induce a thermally stable gel phase in the unitary LacCer bilayer. In LacCer/Cho binary membranes, Cho gradually destabilized the LacCer gel phase to form the liquid-ordered (Lo) phase by its potent order effect. In the LacCer/POPC binary systems without Cho, the ²H NMR spectra of 10',10'-d₂-LacCer and 18',18',18'-d₃-LacCer probes revealed that LacCer was poorly miscible with POPC in the membranes and formed stable gel phases without being distributed in the liquid crystalline (Ld) domain. The lamellar structure of the LacCer/POPC membrane was gradually disrupted at around 60 °C, while the addition of Cho increased the thermal stability of the lamellarity. Furthermore, the area of the LacCer gel phase and its chain order were decreased in the LacCer/POPC/Cho ternary membranes, while the Lo domain, which was observed in the LacCer/Cho binary membrane, was not observed. Cho surrounding the LacCer gel domain liberated LacCer and facilitated forming the submicron- to nano-scale small domains in the Ld domain of the LacCer/POPC/Cho membranes, as revealed by the fluorescence lifetimes of trans-parinaric acid (tPA) and tPA-LacCer. Our findings on the membrane properties of the LacCer domains, particularly in the presence of Cho, would help elucidate the properties of the LacCer domains in biological membranes.

2'-hydroxy C16-ceramide induces apoptosis-associated proteomic changes in C6 glioma cells

Ceramide is a bioactive sphingolipid involved in regulation of numerous cell signaling pathways. Evidence is accumulating that differences in ceramide structure, such as N-acyl chain length and desaturation of sphingoid base, determine the biological activities of ceramide. Using synthetic (R)-2'-hydroxy-C16-ceramide, which is the naturally occurring stereoisomer, we demonstrate that this ceramide has more potent pro-apoptotic activity compared to its (S) isomer or non-hydroxylated C16-ceramide. Upon exposure to (R)-2'-hydroxy-ceramide, C6 glioma cells rapidly underwent apoptosis as indicated by caspase-3 activation, PARP cleavage, chromatin condensation, and annexin V stain. A 2D gel proteomics analysis identified 28 proteins whose levels were altered during the initial 3 h of exposure. Using the list of 28 proteins, we performed a software-assisted pathway analysis to identify possible signaling events that would result in the observed changes. The result indicated that Akt and MAP kinase pathways are among the possible pathways regulated by (R)-2'-hydroxy-ceramide. Experimental validation confirmed that 2'-hydroxy-ceramide significantly altered phosphorylation status of Akt and its downstream effector GSK3β, as well as p38, ERK1/2, and JNK1/2 MAP kinases. Unexpectedly, robust phosphorylation of Akt was observed within 1 h of exposure to 2'-hydroxy-ceramide, followed by dephosphorylation. Phosphorylation status of MAPKs showed a complex pattern, in which rapid phosphorylation of ERK1/2 was followed by dephosphorylation of p38 and ERK1/2 and phosphorylation of the 46 kDa isoform of JNK1/2. These data indicate that (R)-2'-hydroxy-ceramide regulates multiple signaling pathways by affecting protein kinases and phosphatases with kinetics distinct from that of the extensively studied non-hydroxy-ceramide or its unnatural stereoisomer.

Viral attachment induces rapid recruitment of an innate immune sensor (TRIM5α) to the plasma membrane

TRIM5α (tripartite motif 5α) acts as a pattern recognition receptor specific for the retrovirus capsid lattice and blocks infection by HIV-1 immediately after entry. However, the precise mechanisms underlying this rapid recognition of viral components remain elusive. Here, we analyzed the influence of viral exposure on TRIM5α. Total internal reflection fluorescence microscopy and lipid flotation assays revealed rapid recruitment of a TRIM5α subpopulation to the plasma membrane (PM) upon exposure to vesicular stomatitis virus-G-pseudotyped HIV-1 viral-like particles (VLPs), but not to envelope (Env)-less HIV-1 VLPs. TRIM5α signals were frequently colocalized with those of HIV-1 capsid at the PM. Exposure to HIV-1 Env-pseudotyped HIV-1 vectors also triggered translocation of endogenous TRIM5α to lipid microdomains within human T cells. Similarly, clustering of lipid microdomains by a glycosphingolipid stereoisomer resulted in rapid TRIM5α recruitment to the PM. Of note, recruitment of endogenous rhesus TRIM5α to the PM prior to HIV-1 infection significantly increased the potency of viral restriction. Our data therefore suggest the importance of TRIM5α recruitment to the PM for TRIM5α-mediated innate immune sensing and restriction of retroviral infection.

Inhibition of retroviral replication by members of the TRIM protein family

The TRIM protein family is emerging as a central component of mammalian antiviral innate immunity. Beginning with the identification of TRIM5α as a mammalian post-entry restriction factor against retroviruses, to the repeated observation that many TRIMs ubiquitinate and regulate signaling pathways, the past decade has witnessed an intense research effort to understand how TRIM proteins influence immunity. The list of viral families targeted directly or indirectly by TRIM proteins has grown to include adenoviruses, hepadnaviruses, picornaviruses, flaviviruses, orthomyxoviruses, paramyxoviruses, herpesviruses, rhabdoviruses and arenaviruses. We have come to appreciate how, through intense bouts of positive selection, some TRIM genes have been honed into species-specific restriction factors. Similarly, in the case of TRIMCyp, we are beginning to understand how viruses too have mutated to evade restriction, suggesting that TRIM and viruses have coevolved for millions of years of primate evolution. Recently, TRIM5α returned to the limelight when it was shown to trigger the expression of antiviral genes upon recognition of an incoming virus, a paradigm shift that demonstrated that restriction factors make excellent pathogen sensors. However, it remains unclear how many of ~100 human TRIM genes are antiviral, despite the expression of many of these genes being upregulated by interferon and upon viral infection. TRIM proteins do not conform to one type of antiviral mechanism, reflecting the diversity of viruses they target. Moreover, the cofactors of restriction remain largely enigmatic. The control of retroviral replication remains an important medical subject and provides a useful backdrop for reviewing how TRIM proteins act to repress viral replication.

Cytoplasmic body component TRIM5{alpha} requires lipid-enriched microdomains for efficient HIV-1 restriction

TRIM5α is a member of the tripartite motif (TRIM) family of proteins and affects both early and late phases of the retroviral life cycle. Although TRIM5α multimerizes to form cytoplasmic bodies, which are thought to play an important role in viral restriction, the identity of TRIM5α-containing cytoplasmic bodies remains elusive. To better understand TRIM5α cytoplasmic body constituents and the cellular proteins that could be involved in the TRIM5α-mediated antiviral activities, we sought TRIM5α-binding factors. We identified a lipid microdomain protein flotillin-1/Reggie-2 as an interacting partner of TRIM5α via co-immunoprecipitation. Immunohistochemistry studies confirmed the co-localization of rhesus monkey TRIM5α (TRIM5αrh) cytoplasmic bodies with flotillin-1/Reggie-2. Caveolin-1, another lipid microdomain-associated protein, also co-localized with TRIM5α cytoplasmic bodies. Intriguingly, disruption of cellular cholesterol by cyclodextrin perturbed TRIM5α cytoplasmic body formation. Furthermore, lipid starvation partially relieved the endogenous post-entry restriction of HIV-1 infection, which could be subsequently restored by lipid repletion. These observations indicate the involvement of cellular lipids in TRIM5α-mediated antiviral activities. Given that many viruses utilize cellular lipid microdomains for viral entry and assembly, it is plausible that lipid-enriched domains provide microenvironments where TRIM5α recognizes retroviral components.

A lipid matrix model of membrane raft structure

Domains in cell membranes are created by lipid-lipid interactions and are referred to as membrane rafts. Reliable isolation methods have been developed which have shown that rafts from the same membranes have different proteins and can be sub-fractionated by immunoaffinity methods. Analysis of these raft subfractions shows that they are also comprised of different molecular species of lipids. The major lipid classes present are phospholipids, glycosphingolipids and cholesterol. Model studies show that mixtures of phospholipids, particularly sphingomyelin, and cholesterol form liquid-ordered phase with properties intermediate between a gel and fluid phase. This type of liquid-ordered phase dominates theories of domain formation and raft structure in biological membranes. Recently it has been shown that sphingolipids with long (22-26C) N-acyl fatty acids form quasi-crystalline bilayer structures with diacylphospholipids that have well-defined stoichiometries. A two tier heuristic model of membrane raft structure is proposed in which liquid-ordered phase created by a molecular complex between sphingolipids with hydrocarbon chains of approximately equal length and cholesterol acts as a primary staging area for selecting raft proteins. Tailoring of the lipid anchors of raft proteins takes place at this site. Assembly of lipid-anchored proteins on a scaffold of sphingolipids with asymmetric hydrocarbon chains and phospholipids arranged in a quasi-crystalline bilayer structure serves to concentrate and orient the proteins in a manner that couples them functionally within the membrane. Specificity is inherent in the quasi-crystalline lipid structure of liquid-ordered matrices formed by both types of complex into which protein lipid anchors are interpolated. An interaction between the sugar residues of the glycolipids and the raft proteins provides an additional level of specificity that distinguishes one raft from another.